+1
XnView.
used Irfanview in the distant past (not sure about its current state of development), but XnView is more powerful and professional, without being more complicated to use. Even as a lightweight image viewer (including raw) it tops any other.
When you need to retouch images, instead of just resampling, resizing or filtering them, I recommend
Paint.net. Unless you need a more complicated program like
GIMP or Photoshop.
PS I'll split these interesting posts into a new thread in the Technical board :-)